Although measles vaccine has been licensed since 1963 the disease has not been eradicated and it is a significant cause of morbidity and mortality especially in developing countries. The highest incidence of death due to measles occurs in the first two years of life with the highest case-fatalit rates in children under 12 months of age. To delay measles vaccination until 15 months of age in these children would allow a substantial percentage of the morbidity due to measles to continue. One of the obstacles to effective immunization at an early age is the presence of passively acquired maternal antibody. Therefore, an effective measles immunization strategy needs to administer vaccine early enough to be effective yet at an age when maternal antibody would not be expected to inhibit or interfere with the development of a protective immune response. In order to investigate the effect of measles strain and age of immunizatio on the immune response to measles vaccine a double-blind placebo controlled study was done to compare the immune response to two strains of measles vaccine. 174 children were enrolled in the study; 99 children received high titered EZ vaccine at 6 months and 75 children received a standard dose of Schwarz vaccine at 9 months. Of the children who received EZ vaccine 97% o the seronegative children and 85% of the seropositive children showed a four-fold or greater antibody response to measles virus as determined by plaque reduction neutralization assay. Similarly, 97% and 100% of the seropositive and seronegative children,respectively, who received Schwarz vaccine seroconverted. Greater than 90% of children in each vaccine group had protective levels of measles antibody at 3 and 6 months after immunization although the GMT of measles antibody following immunization with the Schwarz vaccine was about two fold greater than antibody levels following immunization with the EZ vaccine. Currently, high dose EZ vaccin is not recommended for use in young children, however this study indicates that protective titers can be achieved in >90% of infants using standard Schwarz vaccine at 9 months.